Spirally wound tubes are used in a variety of applications in which radially inward compressive forces are imposed on the outside diameter (“OD”) of the tubes. For example, continuous materials such as paper, plastic film, metal sheet, and textiles are commonly wound about winding cores formed of spirally wound paperboard tubes. The winding tension required for winding a stable roll of such materials results in substantial compressive forces being exerted by the wound material on the tube in the radially inward direction. Such forces are in a direction to tend to force the inner diameter (“ID”) of the tube to shrink in size. This phenomenon has been referred to as “ID comedown.”
The degree to which a given paperboard tube resists such inner diameter reduction under a given load is referred to herein as the ID stiffness of the tube. The ID stiffness may be expressed as the amount of radially inward uniform compressive pressure on the tube's OD that the tube can withstand for a given amount of ID reduction; thus, for instance, the ID stiffness may have units of psi per inch of inner diameter reduction.
In web winding applications, it is desirable to have a high ID stiffness so that the tube can readily be removed from a winding apparatus after a roll of web material is wound onto the tube. A winding apparatus typically includes some type of chuck or mandrel that is inserted into the tube and is radially expanded to grip the core from the inside. If the tube inner diameter shrinks too much as a result of the forces imposed by the wound material, it can be difficult or impossible to remove the tube from the winding apparatus without destroying the tube.
It is also desirable to have a high OD stiffness. The OD stiffness of a core is the degree of resistance of the core to growth in outside diameter caused by radially-outward pressure exerted on an inner surface of core, such as by expandable winding chucks or mandrels. Such OD growth can lead to problems, particularly when the chucks or mandrels are retracted and the OD shrinks back toward its original size. This can cause a loss of tension in the inner layers of the wound roll of material, which can lead to loss of roll stability, particularly for slippery materials such as sheet metal. It is desirable in many cases to maximize the OD stiffness of a core.
The assignee of the present application has previously discovered that the core's ID stiffness and/or OD stiffness can be increased by forming the core wall to have a radially central region whose compliance in the radial direction is increased relative to that of the core wall regions lying radially inward and radially outward of the central region. See, for example, U.S. Pat. No. 5,505,395, incorporated herein by reference. In the '395 patent, this increased compliance was achieved by using paperboard plies of lower density and strength in the central region of the wall relative to the density and strength of the plies lying radially inward and outward of the central region. Also see, for example, U.S. Pat. No. 6,851,643, incorporated herein by reference. In the '643 patent, this increased compliance was achieved by intentionally introducing wide ply gaps into one or more plies of the central region.
While the approaches represented by the '395 and '643 patents are effective in enhancing the ID stiffness of tubes, it would be desirable to be able to achieve even greater gains in ID stiffness, and to do so in a cost-effective manner.